1. Field of the Invention
The present invention relates generally to devices for filtration of fluids, and in particular to an apparatus for filtering fluids through the use of a casing defining a chamber of variable cross-sectional area along the length thereof for receiving a bed of stratified particulate filtering media.
2. Discussion of the Prior Art
It is well known to filter water and other fluids by passing it through a bed of porous medium, such as sand, anthracite or a combination of both. Filtration is used to produce water of suitable quality for drinking and most industrial uses, including process fluid filtration, waste water treatment, water re-use, gas cleaning, boiler makeup or special processing cooling purposes.
The purpose of filtration is to remove particulate matter from the fluid. More sophisticated filtration systems are needed where it is desired to remove the highest percentage of particulate matter. Filtration involves adsorption, straining, sedimentation, interception, diffusion and inertial compaction.
Conventional filters may be of either gravity or pressure construction. In most such filters, the flow of fluid is downward. Sand or anthracite is usually the filter medium in a bed consisting of either one or two grades of sand or anthracite, with a total depth of 15 to 30 inches. Further, a gravel bed supports the filter medium, and prevents fine sand or anthracite from passing into the underdrain system and distributes backwash water. The supporting bed consists of 1/8 to 11/2 inch gravel in graded layers to a depth of 12 to 16 inches.
One variant of the conventional filter is a stratified upflow filter. In stratified upflow filters, a single medium of graded sand is often used, with the finest sand grade at the top of the bed, and the coarsest sand below. Gravel is generally located at the bottom of the unit retained in a fixed position by grids. The gravel is used to ensure proper water distribution during the service cycle. During operation, larger, coarse particles are removed at the bottom of the bed, while smaller particles are allowed to penetrate further into the media. Typical service flow rates are 5 to 10 gpm/sq.ft.
Conventional fluid filters of the type described above perform adequately in most environments, but experience common problems. One of the principal problems which must be addressed by all filters of this type is that, while the obvious purpose of the filter is to trap as much particulate matter as possible through the initial contact of the water with the filtration media, at the same time some penetration of the media is absolutely essential to prevent rapid loss of pressure head. In other words, in order to promote efficiency, it is beneficial to strike the most positive balance between the highest rate of flow and the lowest loss of pressure head on the one hand, and the highest degree of effluent quality on the other. Conventional filters frequently have difficulty in achieving a high degree of efficiency.
A second problem, related to the first, is created by the need to backwash the filter system in order to remove trapped particulate matter. Because of the relative inefficiency of many conventional filtration systems, it is necessary to backwash such systems at short intervals in order to remove the particles and restore the filter to its maximum efficiency. This frequent backwashing further reduces efficiency by removing the filtration system from its filtering cycle.
Yet another difficulty is presented by the backwashing itself. That is, each backwashing cycle causes fluidization of the particulate media, causing the smaller media to migrate within the chamber and mix with the larger media. This migration is aggravated by the need for frequent backwashing in less efficient conventional filters. The net result of the mixing is frequently to upset the stratification of the bed, thus further decreasing the efficiency of the filter.